I just started a Ldap server and began to autenticate some users for
test (login autentication)

I imported the users and root account from the /etc/passwd and
/etc/shadow (using migrationtools)

When I connect with an existing user or when I create a user in LDAP
and try to login everthing works well, however I can not change to the
root account because I alway got a password error.

Could somebody help me please ?

are you using nss_ldap and pam_ldap? are you using authconfig?? I had
the same pb and found the solution in the "SAmba-LDAP-PDC howto" of
IDEALX (samba.idealx.com)
If you're in the same configuration than I am, you have to change a line
in /etc/pam.d/system-auth: